1. Field of the Invention
The present invention relates to a method of processing an image signal, and more particularly to a method of converting an image signal such as R (red), G (green), B (blue) signals generated by an imaging device such as a color digital camera (also referred to as a digital camera) and representing an original scene which is captured by the imaging device, or an imaging signal such as R, G, B signals generated by an image reading device such as a color scanner (also referred to as a scanner), into a colorimetric signal, processing a colorimetric signal for setup, converting a colorimetric signal processed for setup into a dye density signal for use on a reversal medium, or converting a colorimetric signal processed for setup into c, m, y, k (cyan, magenta, yellow, and black) signals for being supplied to an image output device such as a film producing device, a plate producing device, a color printer, or the like.
2. Description of the Related Art
Device-dependent image signals such as R, G, B signals generated by color digital cameras are converted into device-independent image signals such as tristimulus signals X, Y, Z according to color conversion processes disclosed in Japanese laid-open patent publications Nos. 2-291777 and 2-291778, for example.
Techniques for predicting reproducible colors on prints, for example, device-independent tristimulus signals X, Y, Z from device-dependent c, m, y halftone dot % signals or reproducible colors based on color signals c, m, y, k are disclosed in Japanese laid-open patent publications Nos. 4-337965 and 4-337966, for example.
According to a process of accurately converting colors as disclosed in the Journal of the Institute of Image Electron, Vol. 18, No. 5 (1989), a three-dimensional (XYZ) color space is divided into 512 cubic regions, and color correcting values at a total of 729 grid points are calculated in a process optimized for the characteristics of an output device and stored as the data of a lookup table (LUT). Input values between the grid points are determined by an interpolating process which three-dimensionally interpolates the LUT.
According to another known process, R, G, B signals produced from a color reversal subject (prepared by exposing a color reversal film to light from an image, developing the image on the color reversal film) which carries a positive image are converted into equivalent neutral density (END) signals by a color scanner which is a linear scanning reader, and the END signals are converted into halftone dot % signals with reference to setup points (highlight and shadow points) determined by a setup process. The halftone dot % signals are then converted into c, m, y, k signals, which are then binarized (i.e., converted into halftone dots), finally producing a printing plate or a printed material.
With conventional color scanners, R, G, B signals are processed for setup based on color-separating default conditions (image processing conditions including at least a gradation conversion process, a color correction process, an under color removal process, and a K-plate generating process) which are carried on the respective color scanners and empirically optimized, and R, G, B signals produced from a color reversal subject are converted into c, m, y, k signals.
With respect to R, G, B signals produced by a digital camera, color-separating default conditions may be established using an image processing tool (software) such as Adobe Photoshop (registered trademark).